Set a rack in the middle of the oven and preheat the oven to 350°F. Line the bottom of a 9-inch square baking pan with parchment paper.In a large mixing bowl, combine the butter, graham cracker crumbs, and salt; stir to mix well. Press the crumbs into the bottom of the baking pan. Layer on the chopped nuts, then the semisweet chocolate chips, white chocolate chips, and bittersweet chocolate chips. Sprinkle the coconut evenly on top. Lastly, drizzle on the condensed milk, covering everything with a thin layer.

Bake until golden brown, about 25 minutes. Let cool completely in the pan, then cut into 1½-inch bars. Store in an airtight container.